Control of the biennial thistle, Onopordum, by amitrole and five perennial grasses

Abstract

Experiments carried out from 1959 to 1967 Rear Goulburn in the southern tablelands of New South Wales were designed to test the relative effectiveness of the perennial grasses, Bromtls inermis C.P.I.7073, Dactylis glomerata CV. Brignoles, Festuca arumdinacea CV. Demeter, Lolium perenne CV. Victorian, and Phalaris tuberosa CV. Australian in seasonal control of the biennial thistle Onopordum. The grasses were sown in autumn and in spring. Amitrole (0.5 lb active ingredient an acre) was used to suppress the thistles in the first season. Harvests of the autumn- and spring-sown plots in December 1959 showed large decreases in yield of thistles and increases in yield of grass due to spraying, but in two or three years these effects had disappeared. Dactylis glomerata and Lolium perenne were the most productive grasses at the December 1959 harvest, but the former gave better thistle control. Harvests of the autumn-sown plots in October 1963 showed that Dactylis glomerata and Festuca arundinacea gave the best control of thistles, while in 1966 Dactylis glomerata had almost disappeared and was no longer effective. In 1966, Festuca arumdinacea and Phalaris tuberosa were the only grasses giving effective thistle control. Harvests of the autumn-sown plots in October 1960 showed an increase in yield of the sown grasses but not of the thistles to 30 and 60 lb nitrogen applied in August 1960 before active spring growth of thistles began. Despite a stronger growth of thistles in the first season, the spring-sown grasses showed essentially the same pattern of thistle control as the autumn-sown grasses throughout the remainder of the trial period. Root distribution studies in 1961 showed that Dactylis glomerata had a much higher proportion of its roots in the top six inches than either Festuca arundinacea or Phalaris tuberosa. The greater surface rooting habit of Dactylis glomerata was considered to be important in determining its early success in thistle control, its exclusion of Trifolium subterraneum and its apparent susceptibility to a drought experienced in 1964-65. On the other hand, the deeper rooting habit of Festuca arundinacea and Phalaris tuberosa was taken to explain their persistence and long-term success in thistle control.